If you smoke in the apartment, it is not unusual for the walls to turn yellow after some time ([bei uns wars vor allem hinter den Möbeln]). However, I would repaint first afterwards.
I think your landlord just wants to rip you off. After the short time you have spent in this apartment, no wall turns yellow. That only happens after many more months.
The thing with the 600 euros is exaggerated, I would confront an expert about that. But repainting is probably unavoidable, but it's also normal that the landlord wants that.
That sounds like a rip-off, but repainting is definitely necessary because it is a fact that smoking makes the walls yellow and they have to look clean again when moving out.
It does not matter how long you stay in the rooms, but how much you smoke during that time. Once thoroughly coated with solvent-based renovation paint, the ordeal is over. Buy paint from a specialized retailer or the master painter, not at the hardware store.